The present invention relates to television systems comprising on transmission a camera and on reception a picture restoration device.
The presently known television systems are based on the analysis of images by successive lines scanned from top to bottom at a rate of 50 fields per second in Europe and 60 fields per second in the U.S.A. and Japan. In Europe the number of lines per field is 312.5 or 625 lines for two successive fields as a result of a displacement imposed on the vertical scanning from one field to the next. The same principle is used in the U.S.A. with 262.5 lines per field or 525 lines for two successive fields.
Although these systems are perfectly satisfactory and are utilized to the maximum of their possibilities, they are limited from the performance standpoint mainly with regard to definition when compared with cinematographic projection (for 16 and in particular 35 mm films).
Moreover, there is now a demand for watching television pictures on large screens for which the viewing conditions are different (more open viewing angle, average viewing distance approximately three times the diagonal of the screen instead of five) so that it is necessary to increase the definition of the projected pictures.
Large diameter cathode tubes and projectors able to give the necessary definition have been produced (e.g. "eidophor"), but these devices are both costly and cumbersome.
A high definition television system would appear to be necessary for introducing television methods in shooting intended for the cinema. Thus, the technical means developed for television at present make it possible to produce programmes much more cheaply (immediate display during filming), whilst much more sophisticated trick effects are possible.
In the case of conventional systems it is known to improve the subjective quality of pictures in near vision by a low amplitude, very high frequency modulation of the conventional vertical scanning signal of the receiving tube--this frequency being more than twice as high as the highest frequency transmitted. This is intended to fill the gap between the lines, which is prejudicial at short distance. Although this is effective, it does not restore the lack of definition on shooting.
Another known method consists of producing high definition television systems from a simple extrapolation of conventional systems by increasing the number of lines at scan and on reception (e.g. 1250 line television system). However, these extrapolated systems involve the use of an analog pass band or a digital flow rate which are too high to benefit from the advantages of this method.
Television systems are also known (French Patent Application FR-A 2 142 975), whose principle consists of an "oscillatory" instead of a "linear" horizontal scan of the image. For this purpose scanning is carried out by a spot which not only is subject to the standard deviations, but also to an additional deviation, whose frequency is high compared with the frequency of the scanning lines. This supplementary deviation is called spot wobbling and can be carried out with different predetermined wave shapes, namely sine or square waves. On reception such television systems use scanning signals which are identical to those of the scan so that the arrangement of the displayed points is the same as that of the points scanned on transmission. Thus, the definition of the images in the vertical direction is subjectively improved by a staggered engagement of the displayed points (due to the natural tendency of the eye to seek alignments of points in images).